The present invention relates generally to the continuously variable transmission art. More particularly, the present invention relates to clutch control in continuously variable transmission (CVT) systems in which fluid flow is used to actuate one or more clutches. Specifically, the present invention provides an improved technique, including means and method, for clutch control in a fluid actuated CVT system.
Generally, continuously variable transmissions (CVT's) are well known in the art. A particularly popular version of the CVT system utilizes two adjustable pulleys, each pulley having at least one sheave which is axially fixed and another sheave which is axially movable relative to the first sheave. A flexible belt of metal or elastomeric material intercouples the pulleys. The interior faces of the pulley sheaves are beveled or chambered. As the axially displaceable sheave moves relative to the fixed sheave on each of the pulleys, the distance between the sheaves and thus the effective pulley diameter may be adjusted. The displaceable sheave typically includes a fluid constraining chamber for receiving fluid to move the sheave and thus change the effective pulley diameter; as fluid is exhausted from this chamber, the pulley diameter changes in the opposite sense. Generally, the effective diameter of one pulley is adjusted in one direction as the effective diameter of the second pulley is varied in the opposite direction, thereby affecting a change in the drive ratio between an input shaft coupled to an input pulley and an output shaft coupled to an output pulley. The ratio changes continuously as the pulley diameters vary. The input shaft typically connects to a drive source, such as an automobile engine; the output shaft typically connects to the drive line of the vehicle. A clutch is typically connected in the drive line to control the transfer of torque to the vehicle wheels. Such transmissions, thus, are referred to in the art as continuously variable transmission systems, or CVT's.
As is also known in the art, an hydraulic system typically controls the transfer of fluid in the CVT system. In addition to controlling actuation of the pulley sheaves, the fluid in the CVT system may be further regulated to control actuation of the clutch. Of course, actuation of the clutch controls transfer of torque from the engine, in a ratio controlled by the ratio of the pulleys, to a drive line connected to vehicle wheels to effect movement of the vehicle. Various clutch control techniques for effecting selective actuation of the clutch to provide a desired torque transfer in a number of different operation modes are known in the art.
The present invention relates generally to the same field of endeavor as that of U.S. Pat. No. 4,793,454, entitled "Continuously Variable Transmission Clutch Control System", and U.S. Pat. No. 4,648,496, entitled "Clutch Control System for a Continuously Variable Transmission." Both of these patents are assigned to the assignee of the present application. These patents generally describe logic techniques for regulating pressure at a clutch in a CVT system to provide the desired torque transfer from an engine to a vehicle drive line. Typically, clutch control depends on logical recognition of one of a number of operating modes.
The teachings of each of the above mentioned patents are incorporated herein by reference in terms of background to the present invention, although practical problems may arise in the implementation of the previously envisioned systems in particular applications.